I decided to install xfce as the default desktop, but the documentation is limited. I want to first change the login screen theme (where you put your username and password), then I want to change the icons and then wish that automatically change the desktop wallpaper.

I hope I can help in Gnome 2 was easier to do this, but xfce is a little harder.

Themes are located in /usr/share/lxdm/themeslxdm general configuration file is /etc/lxdm/lxdm.conf fileand the default theme (Sabayon) is looking for its background in /usr/share/backgrounds/, may be you could do some little script to get a random bg each time you boot.